• Can is used to express ability or permission.
Ability: I can swim.
Permission: Can I go out?
Could is the past tense of "can" and also expresses possibility or polite requests.
• Past ability: When I was young, I could run fast.
Possibility: It could rain tomorrow.
Polite request: Could you help me?
• Must indicates strong obligation or necessity.
Obligation: You must wear a seatbelt.
Necessity: I must finish this project today.
• Have to also shows obligation or necessity but is more common in everyday speech.
Obligation: I have to wake up early for work.
Necessity: She has to take medicine daily.